1. Field of the Invention
This invention relates to a method of determining failure of a current detection circuit that detects battery charging and discharging current primarily in a vehicle such as a hybrid car.
2. Description of the Related Art
A hybrid car charges and discharges its batteries while maintaining remaining battery capacity within a prescribed range. This is because over-charging or over-discharging can markedly degrade battery performance. Remaining battery capacity is computed by integrating battery charging and discharging current. Therefore, stable and reliable detection of charging and discharging current is very important in a hybrid car. If it becomes impossible to detect current for integration to obtain battery capacity, estimates of battery capacity become incorrect and batteries can be negatively affected. Battery charging and discharging current is detected by a current detection circuit. Batteries can be charged and discharged normally by determining whether or not the current detection circuit is functioning properly.
Meanwhile, a circuit for determining current detection circuit failure has been developed (Japanese Patent Application Disclosure HEI 9-23501 [1997]).
The prior art circuit mentioned above is provided with a control circuit made up of an inverter circuit to supply current to a three-phase motor, three current sensors to detect current flow through the motor, a tachometer circuit to detect motor rotational speed, a current command generation circuit, a current control circuit, and a pulse width modulation (PWM) signal generation circuit to issue PWM signals. The control circuit executes control of motor operation. The prior art circuit is also provided with a means to determine sensor malfunction, which judges whether a value computed from detected current meets a specified value and issues an error signal depending on that outcome. Further, the prior art circuit is provided with a sensor diagnostic means, which supplies diagnostic current to the three-phase motor based on the error signal and determines if the current sensors are malfunctioning or operating properly depending on detected diagnostic current. Thus, the prior art disclosure describes a circuit configured to execute control of motor operation corresponding to both normal and abnormal sensor operation.
The apparatus described above determines if current sensors are functioning normally or abnormally while current is being supplied to the motor. For example, in a hybrid car, circuits for supplying diagnostic currents and detection of those currents are normally not installed. This is because the circuitry would become complex and cost would be increased. Incidentally, a method of detecting current detection circuit offset to eliminate that offset is well known. That method utilizes periods when current is clearly not flowing. For example, in a system that has an opening and closing switch mechanism, current does not flow when the switch mechanism is in the open state. Therefore, times when the switch mechanism is in the open state are often used to detect offset. However, a state of no current flow cannot serve the additional purpose of determining failure of a current detection circuit. Failure of the current detection circuit cannot be determined because the current detection circuit outputs 0 V as the detected voltage for no current. Specifically, the current detection circuit outputs the same no-voltage signal when it has failed, or when it is operating properly and current flow is 0 A. For example, a current detection circuit with a wire-harness connected to its output-side will have a 0 V output voltage when a failure, such as wire-harness contact failure or open circuit, occurs. It will also have the same 0 V output voltage when the detected current is 0 A. Consequently, failures, such as those in the wire-harness, cannot be determined in a state of no current flow.